1. Field of the Invention
The present invention relates in general to a DC-DC converter for use in a CDI (Capacitor Discharge Ignition) unit for automobiles, two-wheeled vehicles, or outboard motors, and more particularly to suppression of remarkable increase and decrease in output due to the fluctuation of a power source voltage.
2. Description of the Related Art
A separately excited DC-DC converter using an oscillation power MOS-FET in switching on the primary side of a transformer is used as a boosting circuit for charging an ignition capacitor with electricity in an ignition device for charging one pole of the above-mentioned ignition capacitor for accumulating therein the ignition energy which is provided on the primary side of an ignition coil to discharge the electric charges in the capacitor through a primary coil of the ignition coil during the ignition period to induce a high voltage for ignition in a secondary coil of the ignition coil. This DC-DC converter operates to carry out the oscillation and boosting therein by supplying an oscillation signal to a gate of a power MOS-FET from an oscillation circuit provided in order to obtain a desired frequency (e.g. refer to Patent Document 1)
[Patent Document 1]
JP 2927128 B (FIGS. 1 and 4)
However, a power source unit for automobiles, two-wheeled vehicles, or outboard motors can not necessarily supply a stable voltage to a DC-DC converter. Thus, the power source voltage may be increased or decreased depending on the state of the batteries used as the power source unit. In particular, the batteryless system is present in two-wheeled vehicles and hence only the completely unstable power source may be expected in some cases.
While when the power source voltage has been high in the separately excited DC-DC converter using a MOS-FET, the ramp of a drain current ID becomes large, since the MOS-FET is not turned off until a predetermined time is reached, a larger drain current ID is caused to flow through the MOS-FET as compared with the period of the normal power source voltage. In this case, the power of the converter is increased more than is required in the normal operation so that there is increased the danger of bringing about the thermal breakdown or the like of the device.
On the other hand, though when the power source voltage is low, the ramp of the drain current ID becomes small and thus the sufficient drain current ID is not caused to flow through the MOS-FET, the MOS-FET is turned off when the predetermined time is reached so that the output of the converter is remarkably reduced.
In addition, when starting to charge the ignition capacitor with electricity, the switching power MOS-FET is turned on before the energy generated on the primary side is sufficiently transmitted to the secondary side. Thus, during turn-on, some drain current ID is caused to flow through the MOS-FET, whereby during turn-off, the value of the drain current ID becomes larger than that in the normal operation to increase the calorification. Then, if the calorification exceeds the specification of the MOS-FET, the MOS-FET will be broken down.